Question

Consider the process keventd. It spends all it’s lifetime in kernel mode.
Now, as far as I know, Linux checks if a context switch is due, while the process is switching from kernel mode to user mode, and as far as I know, keventd will never switch from kernel mode to user mode, so, how will the Linux kernel know when to switch it off?

Answer 1

If the kernel were to do as you say, and only check whether a process is due to be switched out on an explicit user-to-kernel-mode transition, then the following loop would lock up a core of your computer:

while (1);

Obviously, this does not happen on normal desktop operating systems. The reason why is preemption, where after a process has run for its time slice the kernel gets an alarm, steps in, and forcibly switches contexts as necessary.

Preemption could in principle work for kernel processes too. However, I’m not sure that’s what keventd does – it’s more likely that it voluntarily relinquishes its time slice on a regular basis (see sched_yield, a userspace call for the same effect), especially since the kernel can be configured to be non-preemptible. That is a kernel process’ prerogative.